In recent years, regarding wireless systems, such as mobile phone networks, studies are in progress to install femto base stations (Home eNodeBs (HeNBs)), which form small wireless cells for wirelessly connecting communication terminal devices (User Equipments (UEs)), in ordinary residences, enterprises, and the like in order to improve the qualities of phone services in the installed areas or to enhance service areas.
FIG. 10 is an explanatory diagram illustrating an example of a situation in which signal interference occurs between neighboring HeNBs. A femto wireless system 100 illustrated in FIG. 10 includes a plurality of HeNBs 101 (101A and 101B) and a plurality of Home User Equipments (HUEs)) 102 (102A and 102B). Each of the HeNBs 101 uses the same communication frequency. The HUE 102A is wirelessly connected to the HeNB 101A, and the HUE 102B is wirelessly connected to the HeNB 101B. The HeNBs 101 are located close to each other; therefore, signal interference occurs between the neighboring HeNBs 101. Signal interference also occurs between each of the HeNBs 101 and each of the HUEs 102 that are wirelessly connected to the other neighboring HeNBs 101. Therefore, a communication speed may decrease or communication may be disconnected between the HeNB 101A and the HUE 102A and between the HeNB 101B and the HUE 102B.
As a countermeasure against the signal interference between the neighboring HeNBs 101, a band control process has been known that controls allocation of transmission bands such that the transmission bands to be used do not overlap between the neighboring HeNBs 101. FIGS. 11A to 11C are explanatory diagrams illustrating an example of processing operation performed by each of the HeNBs 101 in relation to the band control process.
The HeNB 101A illustrated in FIGS. 11A to 11C acquires, as illustrated in FIG. 11A, a communication quality, such as a Channel Quality Indicator (CQI) or a Signal Interference Power ratio (SIR), with respect to the wirelessly-connected HUE 102A. The CQI indicates a communication quality on a downlink side, and the SIR indicates a communication quality on an uplink side. Further, the HeNB 101B acquires a communication quality, such as a CQI or an SIR, with respect to the wirelessly-connected HUE 102B. For example, upon detecting degradation of the CQI of a currently-used transmission band on the downlink side on the basis of a CQI value, the HeNB 101A detects signal interference on the downlink side.
The HeNB 101A, upon detecting the signal interference on the downlink side, controls the currently-used band on the downlink side as illustrated in FIG. 11B. The HeNB 101A divides the transmission band on the downlink side into a high band and a low band, turns OFF the transmission power of the transmission band on the high band side, and turns ON the transmission power of the transmission band on the low band side. Consequently, the neighboring HeNB 101B recognizes that the CQI of a transmission band on the low band side is still degraded but the CQI of a transmission band on the high band side is improved in a currently-used transmission band.
The HeNB 101B detects, as an interference difference, a large difference between the CQI of the transmission band on the high band side and the CQI of the transmission band on the low band side in the currently-used transmission band. The HeNB 101B, upon detecting the interference difference in the CQI, recognizes that the neighboring HeNB 101A side has controlled the band. Upon recognizing that the neighboring HeNB 101A side has controlled the band, the HeNB 101B controls the currently-used band on the downlink side as illustrated in FIG. 11C. The HeNB 101B divides the currently-used transmission band on the downlink side, turns ON the transmission power of the transmission band on the high band side in which the CQI is degraded, and turns OFF the transmission power of the transmission band on the low band side in which the CQI is at a high level.
Namely, the HeNB 101A turns OFF the transmission power of the transmission band on the high band side and turns ON the transmission power of the transmission band on the low band side, and the HeNB 101B turns OFF the transmission power of the transmission band on the low band side and turns ON the transmission power of the transmission band on the high band side. Consequently, the transmission bands used by the neighboring HeNBs 101 do not overlap each other, so that it becomes possible to avoid signal interference on the downlink side.
As described above, the HeNB 101, even without a means for notifying the neighboring HeNB 101 of the transmission band to be used, recognizes that the neighboring HeNB 101 has controlled the band on the basis of the communication quality of each of the transmission bands, and autonomously controls the band for the currently-used link. Consequently, it is possible to avoid signal interference on the uplink side or the downlink side.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2010-103753
Non Patent Literature 1: 3GPP TS36.921 v9.0.0
Non Patent Literature 2: 3GPP TS36.922 v9.1.0
For example, when the HUE 102A approaches the neighboring HeNB 101B and the HUE 102A is subjected to signal interference on the downlink side by the HeNB 101B, each of the HeNBs 101 controls a band on the downlink side. FIG. 12 is an explanatory diagram illustrating an example of a situation in which the HUE 102A is subjected to the signal interference on the downlink side by the neighboring HeNB 101B. FIGS. 13A to 13C are explanatory diagrams illustrating an example of operation performed by each of the HeNBs 101 for controlling the band on the downlink side in the situation illustrated in FIG. 12. It is assumed that the HUE 102A is wirelessly connected to the HeNB 101A.
As illustrated in FIG. 13A, the HeNB 101A acquires a CQI report from the HUE 102A, detects degradation of a CQI of a currently-used transmission band on the downlink side on the basis of the CQI, and detects signal interference on the downlink side on the basis of a result of the detection. In contrast, the HeNB 101B acquires a CQI report from the HUE 102B, and because a CQI of a currently-used transmission band on the downlink side is at a high level, recognizes a state in which signal interference on the downlink side is less likely to occur on the basis of the CQI.
The HeNB 101A, upon detecting the signal interference on the downlink side, controls the currently-used band on the downlink side as illustrated in FIG. 13B. The HeNB 101A divides the currently-used transmission band on the downlink side into a high band and a low band, turns OFF the transmission power of the transmission band on the high band side, and turns ON the transmission power of the transmission band on the low band side. However, because an interference difference between the CQI of the transmission band on the high band side and the CQI of the transmission band on the low band side in the currently-used transmission band on the downlink side is minute, the HeNB 101B is unable to recognize that the neighboring HeNB 101A side has divided the transmission band, that is, the neighboring HeNB 101A side has controlled the band.
Accordingly, as illustrated in FIG. 13C, because the neighboring HeNB 101B side does not control the band on the downlink side and therefore the HeNB 101A is unable to recognize that the band on the downlink side is controlled, the HeNB 101A continuously detects degradation of the CQI of the transmission band on the downlink side. Consequently, the HeNB 101A remains in the state in which it is difficult to avoid signal interference on the downlink side.
Further, for example, when the HUE 102A approaches the neighboring HeNB 101B and the HeNB 101B is subjected to signal interference on the uplink side by the HUE 102A, each of the HeNBs 101 controls a band on the uplink side. FIG. 14 is an explanatory diagram illustrating an example of a situation in which the neighboring HeNB 101B is subjected to signal interference on the uplink side by the HUE 102A that is the other one of the HUEs 102. FIGS. 15A to 15C are explanatory diagrams illustrating an example of operation performed by each of the HeNBs 101 for controlling the band on the uplink side in the situation illustrated in FIG. 14. It is assumed that the HUE 102A is wirelessly connected to the HeNB 101A.
As illustrated in FIG. 15A, the HeNB 101B measures an SIR of the transmission band on the uplink side, detects degradation of the SIR of the currently-used transmission band on the uplink side on the basis of the SIR, and detects signal interference on the uplink side on the basis of a result of the detection. In contrast, the HeNB 101A measures an SIR of the transmission band on the uplink side, and because the SIR of the currently-used transmission band on the uplink side is at a high level, recognizes a state in which signal interference on the uplink side is less likely to occur on the basis of the SIR.
The HeNB 101B, upon detecting the signal interference on the uplink side, controls the band on the uplink side as illustrated in FIG. 15B. The HeNB 101B divides, for the HUE 102B, the currently-used transmission band on the uplink side into a high band and a low band, turns OFF the transmission power of the transmission band on the low band side, and turns ON the transmission power of the transmission band on the high band side. However, because an interference difference between the SIR of the transmission band on the low band side and the SIR of the transmission band on the high band side in the currently-used transmission band is minute, the HeNB 101A is unable to recognize that the neighboring HeNB 101B side has divided the transmission band, that is, the neighboring HeNB 101B side has controlled the band.
Accordingly, as illustrated in FIG. 15C, because the neighboring HeNB 101A side does not control the band on the uplink side and therefore the HeNB 101B is unable to recognize that the band on the uplink side is controlled, the HeNB 101B continuously detects degradation of the SIR of the transmission band on the uplink side. Consequently, the HeNB 101B remains in the state in which it is difficult to avoid signal interference on the uplink side.
Namely, in a situation in which the HeNB 101A, as one of the neighboring HeNBs 101, is not influenced by the signal interference, the HeNB 101A is unable to recognize that the HeNB 101B side, as the other one of the neighboring HeNBs 101, controls a band. Therefore, it is difficult to control the bands between the neighboring HeNBs 101, making it difficult to avoid the signal interference between the neighboring HeNBs 101.